1. Field of the Invention
The present invention relates to an acousto-optic tunable filter control apparatus for outputting an optical signal at an arbitrary wavelength from a wavelength division multiplexing (WDM) transmission signal obtained by multiplexing optical signals at different wavelengths, and an optical control method.
2. Description of the Related Art
Recently, demand for an optical communication technique capable of very long distance transmission and high capacity transmission rises following a rapid increase in the number of multimedia network users. To meet this demand, WDM that is a technique for multiplexing optical signals at different wavelengths and for transmitting signals in a plurality of channels to one optical fiber so as to increase a transmission capacity, in particular, is being used.
An optical communication network using the WDM needs to have an optical signal passing-through, splitting, and inserting function and an optical routing and cross connect function. In the former function, the optical communication network passes through, splits, and inserts an optical signal according to a need at each point on the network. In the latter function, the system selects an optical signal at a wavelength in a selected channel from the multiplexed optical signal, and outputs the selected optical signal to an optical transmission path. Accordingly, it is indispensable to do research and development according to a purpose of a user of an optical add-drop multiplexer (OADM) apparatus that passes through, splits, and inserts an optical signal. At present, as OADM apparatuses, a fixed-wavelength OADM apparatus capable of splitting and inserting an optical signal at a fixed wavelength and an arbitrary-wavelength OADM apparatus capable of splitting and inserting an optical signal at an arbitrary wavelength are provided.
As a device that realizes a necessary wavelength selecting function necessary for the OADM apparatus to perform a process for passing through and splitting an optical signal, attention is paid particularly to an acousto-optic tunable filter (hereinafter, “AOTF”). Since a selected wavelength is fixed, the AOTF operates to extract a light at the selected wavelength and can, therefore, select an arbitrary wavelength, differently from fiber grating.
FIG. 9 is an explanatory view of a configuration of the AOTF. As shown in FIG. 9, the AOTF is configured so that two optical waveguides 1-1 and 1-2 are formed on a lithium niobate (LiNbO3) substrate 1-7 that consists of a kind of ferroelectric crystal and that exhibits a piezoelectric action by titanium (Ti) diffusion. These optical waveguides 1-1 and 1-2 intersect each other at two positions, and waveguide polarization beam splitters (hereinafter, “PBS”) 1-3 and 1-4 are provided at the respective intersecting positions.
A SAW guide 1-6 including a metal film is formed on the two optical waveguides 1-1 and 1-2. This SAW guide 1-6 includes an electrode (an inter digital transducer (hereinafter, “IDT”)) 1-5 having interlocking comb tooth. If a radio frequency electric signal (hereinafter, “RF signal”) output from an RF signal generator circuit 1-10 is applied to this IDT 1-5, a surface acoustic wave (hereinafter, “SAW”) is generated. The SAW is propagated in the optical waveguides 1-1 and 1-2.
In FIG. 9, if lights at wavelengths λ1, λ2, and λ3 are input to a port 1 of the AOTF, an input light hybridized in polarization modes of a TE mode and a TM mode by the PBS 1-3 is split into a light in the TE mode and that in the TM mode. The respective lights are propagated in the optical waveguides 1-1 and 1-2. If the SAW is propagated in the SAW guide 1-6 by applying an RF signal f1 at a specific frequency, refractive indexes of the two optical waveguides 1-1 and 1-2 are cyclically changed at intersecting positions, at which the optical waveguides 1-1 and 1-2 intersect the SAW guide 1-6, respectively, by an acousto-optic (AO) effect.
Accordingly, out of the input lights, the polarization mode of the light at a specific wavelength interacting with a cyclic change in the refractive index of each of the optical waveguides 1-1 and 1-2 is rotated, whereby the TE mode and the TM mode are converted into each other. The TE mode is a guided mode in which no electric field component is present in a propagation direction, and the TM mode is a guided mode in which no magnetic field component is present in the propagation direction. A rotation amount is proportional to an action length by which the light in the TE mode or that in the TM mode interacts with the change in the refractive index and a power of an RF signal. The action length is adjusted by a distance between absorbers 1-8 that are formed on the optical waveguides 1-1 and 1-2 across the IDT 1-5 and that absorb the SAW.
Namely, by optimizing the action length and the power of the RF signal, a TM mode light is converted into a TE mode light in the optical waveguide 1-1. In addition, a TE mode light is converted into a TM mode light in the optical waveguide 1-2. A forward direction of the TE mode light or TM mode light thus obtained is changed by the PBS 1-4, and the light at the wavelength interacting with the cyclic change in the refractive index is selected as a split light. FIG. 9 shows that the RF signals f1 and f2 act on the optical signals at the wavelengths λ1 and λ2, respectively, and that the respective lights are selected as split lights.
As explained above, the AOTF can select and split a light at a wavelength according to a frequency of each RF signal. By changing the frequency of this RF signal, the AOTF can change the wavelength of the selected light. In addition, an output light emitted from a port 2 of the AOTF corresponds to optical signals (at wavelength of λ3) obtained by eliminating the lights at the wavelengths corresponding to the frequencies of the RF signals from the input lights incident on the port 1 of the AOTF. Therefore, the AOTF also includes a rejection function.
FIG. 10 is an explanatory view of a relationship between the RF signal and the AOTF output light. In a graph 1000 shown in FIG. 10, a horizontal axis indicates a power of the RF signal (dBm) and a vertical axis indicates a power of the AOTF output light (dBm). As evident from a characteristic curve 1001 shown in FIG. 10, if the RF signal power is changed, the AOTF output light power is changed. While the AOTF is the device that can operate to select a wavelength in response to the RF control signal, the AOTF can be used as a wavelength selection filter that includes a variable attenuation function by using this characteristic.
Techniques using the AOTF as an attenuator are already disclosed (in, for example, Japanese Patent Application Laid-open No. 2002-368317). FIGS. 11 and 12 are explanatory views of variable attenuators using conventional AOTFs, respectively. A variable attenuator 1100 shown in FIG. 11 includes a pump light source 1101, an erbium-doped fiber (EDF) 1102, an equalizer 1103, an AOTF 1104, an RF signal generator 1105, a monitor 1106, and an optical isolator 1107.
The variable attenuator 1100 demultiplexes an output light of the AOTF 1104 to lights at respective wavelengths by a coupler (not shown). Powers of demultiplexed output lights are detected by the monitor 1106. Based on this detection result, intensities (amplitudes) of frequencies f1 to f4 of RF signals applied to the AOTF 1104 are controlled so as to flatten the powers of the output lights at the respective wavelengths. By feedback-controlling the amplitudes of the RF signals based on the powers of the output lights at the respective wavelengths, intensities of SAWs generated according to the respective RF signals are changed. In addition, the powers of the output lights at the respective wavelengths selected by the AOTF 1104 are adjusted according to the intensities of the corresponding SAWs. Therefore, the powers of the output lights at the respective wavelengths are flattened, and the output lights at the respective wavelengths with the flattened power are output to an outside of the variable attenuator 1100.
A variable attenuator 1200 shown in FIG. 12 includes a database 1201 for controlling an RF signal applied to the AOTF 1104 in place of the monitor 1106 included in the variable attenuator 1100. This database 1201 records, as data, correction values of optical powers in all oscillation patterns obtained in advance by measurement or the like. The correction values recorded in the database are referred to according to the respective oscillation patterns, and the intensities of the RF signals applied to the AOTF 1104 are respectively controlled. It is thereby possible to flatten the powers of the output lights.
However, the conventional apparatus that includes the attenuation function similarly to that of the variable attenuator 1100, and that realizes the wavelength selecting function as the AOTF control apparatus has the following disadvantages. If the power of the light at the desired and selected wavelength in the WDM transmission signal input to the AOTF 1104 is high, the power of an initial output light before the light is attenuated by the AOTF 1104 can possibly be set high. If the power of the output light is set high, the optical power of the WDM transmission signal input to an apparatus connected in rear of the AOTF 1104, e.g., an optical receiver can possibly exceed an input power limit of the optical receiver while the variable attenuation function is acting. The light at the power exceeding the input power limit possibly cause damage and a deterioration in the optical receiver.
The power of the input light can be limited so as to prevent a light at an excessive power from being input to the apparatus connected in rear of the AOTF 1104 even before the variable attenuation function acts. However, this restriction makes it difficult to enable an attenuation width of the AOTF 1104 to effectively act. Besides, since the variable attenuation function of the variable attenuator 1100 has a fixed target value set for the powers of the output lights of the AOTF 1104, it is impossible to designate and set an arbitrary attenuation amount according to a user's purpose.
Furthermore, the database 1201 of the variable attenuator 1200 records, as the data, the correction values of the intensities of lights obtained in advance by the measurement or the like. The correction values recorded in the database 1201 are referred to according to the respective oscillation patterns. The intensities of the RF signals applied to the AOTF 1104 are controlled according to the respective correction values, and the powers of the output lights are adjusted by the variable attenuation function. However, the variable attenuator 1200 fails to include a function of compensating for a secular deterioration in the AOTF 1104. As a result, a long-time reliability of the variable attenuator 1200 is not ensured. Besides, since the attenuation characteristics are irregular according to the respective AOTFs, it is necessary to measure and set the values to be recorded in the database in advance for every AOTF. This increases the number of manufacturing steps and imposes a burden on the user. This results in an increase in manufacturing cost.